Image diagnostic systems have been used for diagnosing arteriosclerosis, for preoperative diagnosis upon coronary intervention by a high-performance catheter such as a dilatation catheter (i.e., balloon catheter) or stent, and for assessing postoperative results.
Examples of these image diagnostic systems include intravascular ultrasound (IVUS) imaging systems. In general, the intravascular ultrasound imaging system is constructed to control an ultrasonic transducer to perform radial scanning within a blood vessel, to receive a reflected wave(s) (ultrasound echoes) reflected by biotissue (e.g. the blood vessel wall) by the same ultrasonic transducer, to subject the reflected waves to processing such as amplification and detection, and then to construct and display a tomographic image of the blood vessel on the basis of the intensities of the received ultrasound echoes. An example of such a system is described in JP-A-H06-343637.
In addition to these intravascular ultrasound imaging systems, optical coherence tomography (OCT) imaging systems have been developed in recent years for use as image diagnostic systems. In an OCT imaging system, a catheter with an optical fiber incorporated therein is inserted into a blood vessel. The distal end of the optical fiber is provided with an optical lens and an optical mirror. Light is emitted in the blood vessel while radially scanning the optical mirror arranged on the side of the distal end of the optical fiber, and based on light reflected from biotissue forming the blood vessel, a tomographic image of the blood vessel is then constructed and displayed. An example of this system is described in JP-A-2001-79007.
Improved OCT imaging systems have been proposed in recent years which make use of a wavelength swept light source.
As mentioned above, there are a variety of different image diagnostic systems which use different detection principles. Nonetheless, they are all generally characterized in that a tomographic image (i.e. cross-sectional image) is constructed and displayed by performing radial scanning with a probe. For the construction and display of a high-accuracy tomographic image, it is desirable that a transmission/reception cycle of signals from the probe and a rotation cycle for the radical scanning are in complete synchronization. In general, the rotational speed of a radial scan motor is controlled in synchronization with the transmission/reception repeated at a constant clock in the probe.
The rotational speed of a radial scan motor, however, fluctuates due to variations in torque which occur as a result of changes in the degree of bending of a catheter. Therefore, it is difficult to achieve complete synchronization between the rotational speed of the radial scan motor and the cycle of transmission/reception of signals at the probe.
When a tomographic image is constructed with 1,024 lines by controlling the rotational speed of a radial scan motor, for example at 1,800 rpm (30 Hz), the transmissions/receptions can be performed in accordance with a clock speed of 30.72 kHz. If the rotational speed of the radial scan motor fluctuates by 0.05%, however the number of transmissions/receptions increases or decreases by one transmission/reception in every rotation for radial scanning.
When the number of transmissions/receptions increases or decreases by one transmission/reception in every rotation for radial scanning, the resulting displayed tomographic image is blurred in a circumferential direction or is displayed while slowly turning.